Metal casting processes such as steelmaking, generally utilize metallurgical vessels to transfer and hold molten metal. For example, in steelmaking processes, molten steel is transferred by means of a ladle from the steelmaking vessel to a metallurgical vessel referred to as a tundish. The molten steel is then constantly fed from the tundish into casting molds.
Tundishes are generally made from steel, or a similar metal, and have a working lining that is able to withstand the high temperatures of the molten steel. In order to minimize the tendency of the molten steel to cool and solidify, especially during pouring, the tundish is usually heated to 500.degree.-1250.degree. C. before use. Continuous feeding is accomplished by maintaining a reservoir of molten steel in the tundish. Upon completion of a pouring, any slag or solidified steel remaining in the tundish is scraped from the lining.
In order to protect, and prolong the life of the tundish, and the tundish lining, conventional practice is to cover the lining with a protective layer. It is desirable for the protective layer to adhere well to the working lining and to be substantially impermeable to molten steel and slag. The protective layer should also be disintegratable. The property of disintegrability refers to the tendency of the protective layer to separate immediately behind any slag or solidified steel retained in the tundish at the completion of a pouring to permit the ready removal of the slag or solidified steel without damage to the lining.
The protective layers typical of the prior art include those formed from trowelling materials, "gunning materials" and boards. Trowelling materials used for the protective layer are generally of magnesia, alumina or alumina-silica based refractory aggregate. The material is simply slurried in water and trowelled onto the surface of the tundish lining. Such an operation, however, requires considerable time, skill and labor. Alternatively, a dry product, such as magnesia, alumina, or the like, may be mixed with water and pumped or sprayed ("gunned") onto the tundish.
The protective layer that results from trowelling or gunning contains a considerable amount of water. A tundish having such a protective layer must generally be preheated for from two to five hours to raise its temperature to 500.degree. to 1250.degree. to attempt to drive all the moisture out of the protective layer. This preheating step disadvantageously consumes time, labor and resources.
Tundish boards, also referred to as insulation panels or lagging sheets, are often utilized as tundish linings. The board is generally prepared from a slurry of refractory aggregate, fibrous material and thermosetting resin in water or other liquid. The slurry is drained of excess liquid and formed into a sheet, and the sheet is then oven dried to cure the resin. The boards are generally installed over the refractory lining of a tundish, with the seams between adjacent sheets filled with mortar and covered with a strip of lagging material. The installation of the board is difficult and time consuming. Additionally, the board is easily eroded by the molten steel and slag, and the steelmaking process, particularly at the seams.
In order to overcome the disadvantages of trowelling materials and boards, a monolithic refractory layer may be utilized as protective layer for metallurgical vessels. Canadian Patent No. 1,198,571, issued Dec. 31, 1985 to the Quigley Company, Inc. ("Canadian Patent '571") describes the use of a monolithic refractory layer as a protective layer for a tundish. According to the disclosed method, a monolithic refractory layer is applied within a metallurgical vessel by positioning a mold in the vessel and filling the space between the outer surface of the mold and the inner surface of the vessel with a dry particulate mixture that upon heating, and subsequent cooling, forms the monolithic refractory layer. The mold utilized has an outer surface substantially conforming to the configuration of the inner surface of the vessel so that a substantially uniform space is formed between the outer mold surface and the inner surface of the vessel to form a protective layer of a substantially uniform thickness in the vessel.
The dry particulate mixture disclosed in Canadian Patent '571 consists of at least about 70%, by weight, refractory aggregate, from about 0.5 to 20%, by weight, thermosetting resin and from 0.5 to 10%, by weight, inorganic binder. The preferred dry particulate mixture consists of from about 70 to 99%, by weight, the refractory aggregate having a maximum particulate size of about 5 millimeters, from about 0.5 to 20%, by weight, of the resin and from about 0.5 to 10%, by weight, inorganic binder and from about 0.5 to 10%, by weight inorganic hydrate. The dry particulate mixture is advantageously forced into place between the outer mold surface and the inner surface of the metallurgical vessel using high pressure gas.
The preferred refractory for use in the dry particulate mixture disclosed in Canadian '571 is deadburned magnesia. The preferred resin is a phenol-formaldehyde resin and is cured at 150.degree.-180.degree. C. Sodium silicate, with, or without, magnesium sulfate heptahydrate is disclosed as a suitable inorganic binder.
The monolithic layer disclosed in Canadian Patent '571 has at least the following disadvantages. First, the inorganic binder requires a relatively high temperature in order to bind the other materials in the dry particulate mixture, and may not completely burn out during formation of the protective layer. Burn out refers to the ability of the binder to completely decompose during heating in order to leave the resulting monolithic protective layer substantially free of the binder. Because an inorganic binder may not completely burn out, its residues may react with the refractory lining of the metallurgical vessel and/or act as a flux, both of which are disadvantageous.
Additionally, phenol formaldehyde resins are classified as hazardous and toxic by OSHA, in the U.S., and similar agencies in other countries. The heating of a particulate mixture including a phenol formaldehyde resin may release hazardous, and or carcinogenic gases. Therefore, protective clothing and special handling procedures are necessary to use this type of resin in a protective layer. Thus the use of phenol formaldehyde resins is a major disadvantage of the protective layer described in Canadian '571.
The present invention overcomes the aforementioned disadvantages of previously utilized protective layers, and binder compositions for refractory layers.